1. Field of the Invention
This invention relates to a diffraction optical element, and particularly to a diffraction optical element suitable for use in a light including a plurality of wavelengths, or a wide-band light, and an optical system using the same.
2. Related Background Art
In contrast with a conventional method of decreasing chromatic aberration by a combination of glass materials, a method of providing a diffraction optical element (hereinafter referred to also as the diffraction grating) having the diffracting action on a lens surface or a portion of an optical system to thereby decrease chromatic aberration is disclosed in literature such as SPIE, Vol. 1354, International Lens Design Conference (1990), Japanese Patent Application Laid-Open No. 4-213421, Japanese Patent Application Laid-Open No. 6-324262 and U.S. Pat. No. 5,044,706. This method utilizes the physical phenomenon that in a refracting surface and a diffracting surface in an optical system, chromatic aberration for rays of light of a certain reference wavelength appears in opposite directions. Further, such a diffraction optical element can also be given an effect like that of an aspherical lens by the period of the periodic structure thereof being changed, and has a great effect in reducing aberrations.
Here, in refraction, a ray of light is a ray of light still after refraction, whereas in diffraction, a ray of light is divided into a plurality of orders. So, when a diffraction optical element is used as a lens system, it is necessary to determine the grating structure so that a light beam of a wavelength area used may concentrate in a particular order (hereinafter referred to also as the xe2x80x9cdesign orderxe2x80x9d). When a light concentrates in the particular order, the intensity of the other rays of diffracted light becomes low, and when the intensity is zero, the diffracted light thereof becomes null.
Therefore, when the aberrations of an optical system are to be corrected by a diffraction optical element, it becomes necessary to that element that the diffraction efficiency of a ray of light of the design order be sufficiently high in the entire wavelength area used. Also, when there are present rays of light having the other diffraction order than the design order, those rays of light are imaged at locations discrete from the location for the ray of light of the design order and therefore become flare light. Accordingly, in an optical system using the diffraction effect, it is important to give sufficient consideration also to the spectral distribution of the diffraction efficiency at the design order and the behavior of the rays of light of the other diffraction orders than the design order.
When a diffraction optical element 201 having a diffraction grating 204 comprising a layer provided on a substrate 202 as shown in FIG. 16 of the accompanying drawings is formed on a certain surface, the characteristic of diffraction efficiency for the particular diffraction order is shown in FIG. 17 of the accompanying drawings. Hereinafter, the value of the diffraction efficiency is the rate of the quantity of each diffracted light to the whole transmitted light beam, and is a value in which the reflected light or the like on the boundary surface of the diffraction grating is not taken into consideration because it is complex to describe. In FIG. 17, the axis of abscissas represents wavelength and the axis of ordinates represents diffraction efficiency. The diffraction optical element 201 is designed such that in the first diffraction order (solid line in the figure), the diffraction efficiency becomes highest for the wavelength area used. That is, the design order is the first order. Further, the diffraction efficiency for the diffraction orders in the vicinity of the design order (zero order and second order which are first orderxc2x1one order) is also shown. At the design order, the diffraction efficiency becomes highest for a certain wavelength (hereinafter referred to as the xe2x80x9cdesign wavelengthxe2x80x9d) and gradually becomes lower for the other wavelengths. The decrement in the diffraction efficiency at this design order is the increment in the diffraction efficiency at the other orders than the design order, and diffracted lights of the other orders than the design order become flare lights. Also, when a plurality of diffraction optical elements are used, particularly a reduction in the diffraction efficiency at the other wavelengths than the design wavelength also leads to a reduction in transmittance.
A construction which can decrease such a reduction in the diffraction efficiency is disclosed in Japanese Patent Application Laid-Open No. 9-127322. This, as shown in FIG. 18 of the accompanying drawings, optimally selects three kinds of different materials and two kinds of different grating thicknesses, and disposes them in proximity at an equal pitch distribution to thereby realize high diffraction efficiency in the entire visible area, as shown in FIG. 19 of the accompanying drawings.
Also, the assignee of the application presents in Japanese Patent Application Laid-Open No. 10-133149 a diffraction optical element which can decrease any reduction in diffraction efficiency. FIG. 20 of the accompanying drawings shows the construction presented in the above-mentioned proposition, and it has a laminated cross-sectional shape in which two layers are stacked. High diffraction efficiency is realized by optimizing the refractive indices of materials forming the two layers, the spectral characteristic and the thickness of each grating.
If a diffraction optical element is provided in an optical system, light beams of various angles of views usually enter the diffraction optical element. Therefore, if a diffraction optical element having a diffraction grating provided on a flat plate is used in an optical system, the angle of incidence of a light beam onto the diffraction optical element is changed by the angles of view, and the diffraction efficiency of diffracted light at the design order comes to be changed by the angles of view.
In particular, a laminated diffraction optical element, as compared with the prior-art single-layer diffraction optical element shown in FIG. 16, tends to become greater in the thickness of the grating. Therefore, a laminated diffraction optical element formed on a flat plate, when used in an optical system having an angle of view, has its diffraction efficiency greatly reduced by the eclipse or the like of a light beam on the edge surface of the grating.
In contrast, it would occur to mind to use a diffraction optical element having a diffraction grating on a curved surface in an optical system having an angle of view. In this case, when the diffraction optical element is disposed, for example, more adjacent to the object side than a stop, a change in the angle of incidence of a light beam onto the diffraction optical element may be reduced depending on a change in the angle of view by the diffraction grating being provided on a curved surface which is concave relative to the stop.
Actually, however, unless the shape of the grating when a diffraction optical element having laminated structure is formed on a curved surface is made appropriate, high optical performance equal to that of a diffraction optical element of laminated structure formed on a flat surface will not be obtained in some cases.
The present invention has as its object the provision of a diffraction optical element which, even when used in an optical system having an angle of view, has a small change in diffraction efficiency depending on the angle of view, and an optical system using the same.
A first diffraction optical element of the present invention is a diffraction optical element having structure in which at least two diffraction gratings formed of at least two kinds of materials differing in dispersion are laminated, and having its diffraction efficiency of a particular order enhanced in the wavelength area used, characterized in that at least two of the diffraction gratings are formed on a curved surface and adjacent to each other, and these two adjacent diffraction gratings are of a curved surface shape in which curved tip planes in which the tips of respective grating portions are ranged are equal to each other.
A second diffraction optical element of the present invention is a diffraction optical element having structure in which at least two diffraction gratings formed of at least two kinds of materials differing in dispersion are laminated, and having its diffraction efficiency of a particular order enhanced in the wavelength area used, characterized in that at least two of the diffraction gratings are formed on a curved surface and adjacent to each other, and a curved tip plane in which the tips of the grating portion of one of these two adjacent diffraction gratings are ranged and a groove bottom curved surface in which the groove bottoms of the grating portion of the other diffraction grating are ranged are of an equal curved surface shape.
A third diffraction optical element of the present invention is a diffraction optical element having structure in which at least two diffraction gratings formed of at least two kinds of materials differing in dispersion are laminated, and having its diffraction efficiency of a particular order enhanced in the wavelength area used, characterized in that at least two of the diffraction gratings are formed on a curved surface and adjacent to each other, and these two adjacent diffraction gratings are such that a line linking the tips of the opposed grating portions thereof is substantially parallel to an optical axis.
A fourth diffraction optical element of the present invention is a diffraction optical element having structure in which a plurality of diffraction gratings formed of at least two kinds of materials differing in dispersion are laminated, and having its diffraction efficiency of a particular order enhanced in the wavelength area used, characterized in that two adjacent ones of the plurality of diffraction gratings satisfy
xcex1xe2x89xa6xcex2
where xcex2 represents the angle formed between the grating edge of the grating portion thereof and the grating surface of the grating portion, and xcex1 represents the angle formed by the grating surface with respect to the surface normal of a curved tip plane at a position whereat the curved tip plane in which the tips of the grating portion are ranged and the tips intersect with each other.
A fifth diffraction optical element of the present invention is a diffraction optical element having structure in which a plurality of diffraction gratings formed of at least two kinds of materials differing in dispersion are laminated, and having its diffraction efficiency of a particular order enhanced in the wavelength area used, characterized in that the grating thickness of the grating portions of the diffraction gratings is such that the length of the grating thickness in a direction parallel to the surface normal of a curved tip plane in which the tips of the grating portions are ranged at a position whereat the curved tip plane and the tips intersect with each other is constant.
A sixth diffraction optical element of the present invention is a diffraction optical element having structure in which at least two diffraction gratings formed of at least two kinds of materials differing in dispersion are laminated, whereby the diffraction efficiency of a particular order (design order) is enhanced in the entire wavelength area used, characterized in that at least two of the diffraction gratings are formed on a curved surface and adjacent to each other, and these two adjacent diffraction gratings are equal to each other in the center of curvature of the curved tip plane of each grating portion thereof in which the most proximate tips of the gratings are ranged.
A seventh diffraction optical element of the present invention is a diffraction optical element having laminated grating structure in which at least two diffraction gratings formed of at least two kinds of materials differing in dispersion are stacked in proximity to one another, and having its diffraction efficiency of a particular order enhanced in the wavelength area used, characterized in that two of the diffraction gratings are formed on a curved surface and adjacent to each other, and the grating spacing between these two adjacent diffraction gratings is equal over the range of use.
Each of the aforedescribed elements has a form in which the plurality of laminated diffraction gratings are joined together in the non-grating area of each diffraction grating.
Each of the aforedescribed elements has a form in which the plurality of laminated diffraction gratings include a diffraction grating differing in the direction of the grating shape from at least one other diffraction grating.
Each of the aforedescribed elements has a form in which the wavelength area used is a visible range.
Each of the aforedescribed elements has a form in which at least one of the plurality of diffraction gratings is such that the material forming the diffraction grating is the same as the material forming a substrate on which the diffraction gratings are provided.
Each of the aforedescribed elements has a form in which a substrate on which the diffraction gratings are formed has lens action.
Each of the aforedescribed elements has a form in which it is formed on the cemented surface of a cemented lens.
The optical system of the present invention is characterized in that it uses any one of the aforedescribed elements and the aforedescribed forms of diffraction optical element.
The optical system is an imaging optical system or an observation optical system.
FIG. 1 is a schematic view of the essential portions of a diffraction optical element according to Embodiment 1 of the present invention.
FIG. 2 is a cross-sectional view of the essential portions of a diffraction grating in Embodiment 1 of the present invention.
FIG. 3 is an illustration of the diffraction efficiency of the diffraction optical element according to Embodiment 1 of the present invention.
FIG. 4 is an illustration of grating pitch.
FIG. 5 is an illustration of the curved surface shape of the tip of a diffraction grating.
FIG. 6 is an illustration of the curved surface shape of the tip of a diffraction grating.
FIG. 7 is an illustration of the curved surface shape of the tip of a diffraction grating.
FIG. 8 is an illustration of the grating pitch and curved surface shape of the diffraction grating.
FIG. 9 is an illustration of the grating edge shape.
FIG. 10 is an illustration of the thickness of the grating.
FIG. 11 is an illustration of a one-dimensional diffraction optical element according to Embodiment 1 of the present invention.
FIG. 12 is a schematic view of the essential portions of a diffraction optical element according to Embodiment 2 of the present invention.
FIG. 13 is a schematic view of the essential portions of a diffraction optical element according to Embodiment 3 of the present invention.
FIG. 14 shows a photo-taking optical system according to Embodiment 4 of the present invention.
FIG. 15 shows an observation optical system according to Embodiment 5 of the present invention.
FIG. 16 is an illustration of the grating shape (triangular wave shape) of a diffraction optical element according to the prior art.
FIG. 17 is an illustration of the diffraction efficiency of the diffraction optical element according to the prior art.
FIG. 18 is an illustration of the cross-sectional shape of the diffraction grating of a laminated type diffraction optical element according to the prior art.
FIG. 19 is an illustration of the diffraction efficiency of the laminated type diffraction optical element according to the prior art.
FIG. 20 is an illustration of the cross-sectional shape of the diffraction grating of the laminated type diffraction optical element according to the prior art.
FIG. 21 is an illustration of the cross-sectional shape of a diffraction optical element according to the prior art formed on a flat plate.
FIG. 22 is an illustration of the diffraction efficiency of the diffraction optical element according to the prior art formed on a flat plate.